A lot of people (primarily those who play humans) use places such as www.renderosity.com to search for suitable images. That ensures that you end up with something original, rather than a picture of an actor several other players use as well, for example.
For other races it can be a little bit trickier, but Google's image search can help out.
1) click character button in members area.
2) click edit your character profile link displayed near the top of the page you get to.
3) Edit the url to character profile image. The url would be the location online for your new character image. Make sure that you have a place to store it and don´t remote link it from someone elses site without permission.